Large scale volume lights
An enlarged object set with volume scatter will seem denser, as the camera rays will need to traverse a larger space.
Make the lights brighter or bring down the density of the volume scatter and/or volume absorption.

As alternative use the world volume as domain instead of large volumes.

Another think to keep in mind is the camera clipping distance.  It is possible that if your lights fall outside the camera's clip distance they might have no effect at all.